That’s one of many questions that will have to be answered as Canada moves towards legalizing marijuana and police wonder if they’re equipped to crack down on stoned drivers.

“We’re having our challenges. The most pressing one is that we don’t know what the legislation will look like. It makes it hard to train and prepare,” said Supt. Gord Jones of the Toronto Police, the co-chair of the Canadian Chiefs of Police traffic committee.

We talk to experts about what is known and what is unknown about cannabis-impaired driving — and why detecting and charging drivers under the influence of marijuana will be more complicated than nabbing drunk drivers.

Q: How does roadside drug testing work now?

A: Under legislation introduced in 2008 to update impaired driving laws, drivers suspected of drug use have been required to participate in a drug evaluation conducted by a Drug Recognition Expert, or DRE.

These police officers, trained to an international standard, rely on their observations to determine whether a blood or urine test is warranted.

The problem is that there are fewer than 600 trained DRE officers in Canada. An assessment conducted in 2009 estimated that Canada needs between 1,800 and 2,000 and the training system isn’t equipped to pump out trained officers any faster.

Q: Are drivers under the influence of marijuana really dangerous on the road?

But it’s unclear how much THC it takes to impair a driver, according to the Canadian Centre on Substance Abuse. The effects appear to be individual. One person may be substantially impaired after a relatively small amount of marijuana, while someone else may be only moderately impaired after the same dose.

Q: When it comes to cannabis, what constitutes over the limit?

A: There is no consensus. In other jurisdictions in the U.S. and Europe where marijuana is legal, limits vary widely. Contrast that to drivers who are impaired by alcohol, where breathalyzer tests are generally accepted in court.

“There is no legislation right now where a number means anything. It means nothing in the law,” said Jones. “Until we get a number, we have no number to prosecute.”

Q: Experts believe that a lot of drivers impaired by drugs are going undetected. What’s the evidence for that?

A: Statistics Canada reported almost 75,000 impaired driving incidents in 2015, only abut 2,500 were related to drugs. Sounds like there’s not a lot of drivers impaired by drugs, right? But at the same time, studies have found that a large proportion of drivers who die in crashes have drugs in their systems. Drugs of all kinds, not including alcohol, were found in the systems of more than a third of fatally injured drivers between the ages of 16 and 24, according to a 2010 study, for example.

In Colorado, where medical cannabis was legalized in 2006 and recreational cannabis was legalized in 2013, fatal injuries involving drivers who used cannabis while driving doubled between 2007 and 2012.

“We’re getting a picture that people who are using cannabis are dying in greater numbers than ever before,” said Douglas Beirness, a senior researcher with the Canadian Centre on Substance Abuse.

Q: Why is detecting marijuana impairment different from alcohol impairment?

A: With an alcohol-impaired driver, there’s a easy giveaway for police — the driver smells like booze. But a marijuana user might not necessarily smell like pot. Cannabis users are increasing consuming it as edibles such as cookies, said Beirness.

And even if the driver does smell like pot — or if he or she tested positive after a fatal accident — that doesn’t necessarily mean the cannabis was recently consumed. Drugs metabolize differently from alcohol. Chemical traces of cannabis remain in the body for far longer than alcohol.

Toxicology tests indicate that a drug has been consumed, but unlike a breathalyzer, they don’t indicate how long ago the drug was consumed, said RCMP Sgt. Ray Moos, national co-ordinator of the DRE program.

“Drugs affect everyone differently based on the drug and the way the body metabolizes the drug.”

Q: Will it be difficult to prosecute drivers who are suspected of cannabis impairment?

A: There’s a lot of history in prosecuting alcohol-impaired drivers. But even these charges are sometimes challenged in the courts, so police are braced for court challenges in cannabis-impairment charges.

Police enforce the laws that are given to them by the government, said Jones. “This is something new. There will be challenges until there are decisions in the court.”

Q: Why is it important to have an over-the-limit number?

A: Having a number makes court procedures easier because courts like numbers, said Beirness. That’s the way it has worked with alcohol impairment. Having a number also tends to have a deterrent effect.

Q: How are DREs trained?

A: Training is done in two stages, beginning with a 10-day classroom portion in which trainees are trained in how to recognize the way drugs, both licit and illicit, impair the body. They learn to pay attention to symptoms such as blood pressure and heart rate, said Moos. DRE officers carry a kit containing a stethoscope, blood pressure cuff and thermometer.

After their time in the classroom, the officers do a four-day field certification, where they analyze subjects for impairment. This is done with volunteers in U.S. health clinics and remand centres.

The DRE is typically not the person who stops a suspect driver, but they will be called in these cases. (There were 891 DRE evaluations in Canada in 2013; 1,400 in 2014 and 1,800 in 2015)

While a DRE’s observations are enough to lay charges, a DRE can demand that the individual provide a urine, saliva or blood test at a police station, a process that takes about half an hour. The tests are then sent to a toxicology lab. The RCMP have three labs, and the OPP and QPF also have their own labs.

Q: Why would it be hard to add more DREs to police rosters?

A: There are currently about 578 DREs in Canada, and 160 to 200 new DREs are certified every year. To compare, there were over 7,000 DREs in the U.S. in 2014.

But some existing Canadians DREs do not re-certify or they are promoted out of the role. It’s hard enough to maintain the current number of DREs, much less increase their numbers, said Beirness.

At the same time, training is expensive, and some of it currently has to be done in the U.S. Opportunities to get field training in the U.S. are getting squeezed as demand to train officers increases there.

Canadian police are looking at finding a centre where officers can learn drug evaluation in Canada. But there have been concerns about potential human rights abuses in using detention centres or health clinics for this purpose, said Jones.

A: Not necessarily. The pilot project is looking at whether these tests can be used under Canadian weather conditions. The goal of the project is to determine whether the devices will stand up to the rigours of policing in Canadian weather conditions, said Jones. Seven police forces are using the saliva tests at roadside in the pilot project, including the Gatineau Police.

The devices can be calibrated to “hit” on a particular level of THC – but that level has not yet been determined, points out Jones. Meanwhile, a saliva test will only give probable grounds that the subject has used drugs. They would still need to be seen by a DRE.

The devices can be customized to test for a broad range of drugs, but as it stands they only test for the most common five or six, said Jones.

“It’s not a panacea. If there is no hit, that doesn’t mean the person isn’t impaired.”

The devices are also expensive — a saliva test can cost between $20 to $40 each, compared to pennies for a disposable mouthpiece for a breathalyzer test, said Beirness. Given the costs, it will be up to every police force to determine the most cost-effective way to use them.

Q: What else can be done?

A: Officers can do standardized field sobriety tests, but it’s not something every officer is trained to do. (One test you often see in movies is the officer requiring the driver to walk in a straight line and turn.) Done correctly, field sobriety tests are just as effective for drug users as alcohol users, said Beirness.

Most Canadian DREs demand a urine test rather than a blood test. However, blood tests give a THC concentration level. In the U.S., some police officers are being trained to take blood samples at the roadside.

However, blood tests are rare in Canada because they have to be supervised by a physician. Beirness suggests that it doesn’t need to be a doctor who administers the test, it could be another medical professional, or a police officer who has taken the training, which takes about two days.

Meanwhile, sanctions for impaired driving, commonly used in alcohol cases, act as a deterrent. The driver’s licence may be suspended and their car may be impounded. Loss of freedom is a motivator, Beirness notes.

Q: Is there cause to worry?

A: It appears that legislation legalizing marijuana will be tabled this spring. It will take another 18 months before it comes into effect, Beirness believes.

“The sky’s not falling. But it’s going to be a significant concern.”

He urges more public awareness. “Young people between 15 and 24 don’t think cannabis use has any effect on driving, and that police can’t do anything about it. We have to create a culture that doesn’t accept the use of cannabis and the operation of a motor vehicle.”

By the numbers:

2.6 per cent: Proportion of drivers in Canada who admitted to driving within two hours of using cannabis in the past year, according to Health Canada’s 2012 Canadian Alcohol and Drug Use Survey

632,576: How many people this represents

10.4 million: How many trips this represents

2.04 million: How many Canadian drivers admitted to driving after consuming two or more drinks in the previous hour

13.3 million: How many trips this represents

3X: Likelihood of males to drive after using cannabis compared to females

5.5 per cent: Proportion of drivers who tested positive for cannabis use, according to a 2013 study in British Columbia based on a random sample of nighttime drivers

16.6 per cent: Proportion of fatally injured drivers who tested positive for cannabis, according to an examination of 2000-2010 Canadian data

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